Image storage tube



1941- R. M6L ER arm.

IMAGE STORAGE TUBE Filed Feb. 4; 1959 Patented July. 29, 194i IMAGESTORAGE TUBE Rolf Miiller, Klein Maclmow, and Werner Hartlslimnn,Zelilcndorf,

Zehlendorl, Germany Application near Berlin, Germany, asgnorsto-Fernseli Aktlengescllschaft, Berlin'- February 4, 1939, Serial No.254,592 Germany February 8,1938

. 3 Claims. (01. 250-150) Our invention relates to storage pick-up tubesfor television purp ses, and particularly to such tubes utilizing amosaic of insulated photoelectric elements upon'which a charge image isstored in accordance with an optical image, and in which this mosaic isscanned point by point. Under scansion a picture signal is generated andthe charges on the-mosaic, elements under scansion neutralized,whereupon recharging takes place.

In known tubes, scansion of the mosaic is effected by impact of ahigh-velocity electron beam which causes secondary electrons to beemitted and the charges on the mosaic elements to become neutralized.The potential which the elements will acquire after neutralizationdepends upon the secondary emission produced by the scanning beam andadjusts itself to an equilibrium potential which is only several voltsbelow that of the electrode to which the secondary electrons areaccelerated. Consequently only a 9 weak accelerating field exists infront of a mosaic during the charge and it becomes extremely diflicultto achieve saturation of the photoelectrons under the influence of theincident optical image. Another consequence thereof is that not allsecondary electrons generated at the mosaic by the scanning beam aredrawn away, but a substantial portion thereof is drawn to neighboringmosaic elements and prematurely discharge the latter. This largelyimpairs the efiiciency of the tube as well 'as the sharpness of theimage produced. These defects exist also in known tubes withdouble-sided mosaics. In that case, a strong accelerating field can beused for the photoelectrons. n the side impacted by the scanning beam,however, a strong accelcrating field still cannot be used because theequilibrium potential is determined by the secondary emission.

It is the object of this invention to overcome the aforesaiddisadvantages wholly or in part; to provide a mosaic and to providestrong accelerating fields for both photoelectrons and scanningelectrons. Other objects and features of our invention will become clearfrom the following description.

Referring to the drawing, Fig. 1 schematically illustrates a crosssection through a storage tube according to our invention, reduced tothe simplest terms for sake of simplicity; Fig. 2 schematically shows across section through a mosaic screen embodying our invention; and Fig.3 schematically shows a cross section through a modified type of mosaicscreen according to our invention.

' place. This invention,

screen simple in construction,

Broadly considered, our invention provides the use of a strongaccelerating field in frontof the mosaic and scansion with low-velocityelectrons which liberate no or only a few secondary electrons. Thelow-velocity electrons used for scansion are generatedphotoelectrically. While it is known in the art to produce trons byphotoelectric means, the disadvantage of this method when hitherto usedhas been that the mosaic needed to be double-sided and that a diiiusionof the scanning electrons would take therefore. provides that thephotoelectrically produced scanning electrons impact the mosaic on thesame side from which the picture photoelectrons are liberated, and thatthe fields traversed by the scanning electrons are separated from thefield which accelerates. the picture photoelectrons. By doing so theimpacting velocity of the scanning electrons is determined by thepotential difference between the potential of the mosaic elements indischarged condition and the potential produced by the charges caused byexposure to light. In order to prevent diffusion or spreading of thelowvelocity scanning electrons, the fields traversed by the scanningelectrons are preferably separated for each individual picture element,so that the scanning electrons again impact only the correspondingmosaic elements. For this purpose, all mosaic elements can be screenedoff against each other.

In operation,-an optical image is projected upon the photomosaic.Photoelectrons are accelerated by means of a strong field and the mosaicelements acquire a positive charge,the magnitude of which isproportional to the intensity of light incident thereupon. Underscansion, low-velocity photoelectrons are emitted in the immediateproximity of the mosaic elements under scansion, whereby a portion ofthese low-velocity scanning electrons will .be drawn toward the mosaicelement by virtue of the potential difference betwen the mosaic elementand the emitter of scanning electrons. This portion of scanningelectrons will neutralize the charge on the mosaic elements, while theremainder of the scanning electrons will be used as the picture signal.The invention will now be described in detail in connection with thedrawing.

Fig. 1 shows a vacuum receptacle l housing an electron gun consisting ofa cathode 8, an

anode 6- and two pairs of deflecting plates I.

' Reference numeral 2 designates the mosaic screen upon which an opticalimage is proiected by means of lens 3. A is a wide open wire meshscanning elec-' screen 4 servesas an accelerating electrode or picturesignal collector. Two electron multipliers 5 and 5a, each comprising aplurality of secondary-emissive grids and a solid plate collector, maybe provided. A voltage source 9 serves to heat the cathode 8. A voltagedivided I! in cooperation with a voltage source i8 for supplies anode 6,accelerating screen 4 and electron multipliers 5 and 5a with operatingvoltages. A signal output resistor l9, across which the signal can betaken off by way of a condenser 20, is connected to the anodes of theelectron multipliers 5 and 5a.

Fig. 3 shows a modification of the mosaic screen which includes atransparent insulating layer I I, a fluorescent screen l4, a wire meshscreen l2 and the mosaic electrodes l3 which, in this case, arebutton-like in shape, possessing a photosensitive front surface and sidesurfaces insensitive to light and secondary emission.

Fig. 2 shows a screen comprising two layers H of an insulating material,such as mica, between which a translucent metal layer I6 is disposed.One of the layers II is provided with a fluorescent screen l4, while theother layer II is in close contact with a metal wire mesh screen I2. Inorder to secure a close contact it may be preferable to provide thelayers H and the metal layer l6 with a slight curvature and to stretchthe metal screen l2 across the same. In the openings of the screen i 2photoelectric mosaic elements l3 are deposited upon the last-mentionedof the layers ll. Portions l5 of the screen l2 are made to bephotoemissive.

Fig. 3 shows a modification of the mosaic screen which includes atransparent insulating layer II, a fluorescent screen l4, a wire meshscreen I! and mosaic electrodes l3 which. in this case, are button-likein shape, possessing a photosensitive front surface and side surfacesinsensitive to light and secondary emission.

In operation, an optical image is focused upon the right side of amosaic screen 2, as shown in Fig. l, and photoelectrons emitted bymosaic elements i3 are accelerated toward grid 4, which is heldapproximately 100 volts positive with respect to the assumed potentialof the photomosaic. The accelerating field developed by the potentialapplied to the electrode 4 reaches into the openings of the screen I2and exerts In case some photoelectrons should be intercepted by thescreen l2, the latter rapidly accumulates a negative charge thereby,hence preventing any further collection of photoelectrons from themosaic elements l3 and enhancing the focusing above referred to. Thecathode ray beam produced by cathode 8 which is deflected by applyingsawtooth voltages to deflecting plates 1, as is well-known in the art,is scanned across fluorescent screen l4 and produces light upon impacttherewith. Due to the bombardment by high velocity electrons thefluorescent screen l4 will assume a potential which is positive to thecathode 8. In accordance with standard practice, the portion of theinner wall of the envelop I surrounding the discharge space between thecathode 8 and the fluorescent screen l4 may, if desired, be providedwith a conductive coating (not shown) connected to the anode 6 in orderto collect the secondary electrons emitted by the fluorescent screen l4and anode 6,

and to prevent wall charges. By virtue of electro-static induction thescreen I 2, separated from the fluorescent screen by a layer ofinsulating material, will assume a potential negative with respect tothat of the screen i4.

these photoelectrons will mosaic elements I3, as indicated by the dottedlines in Figs. 2 and 3. This portion of the lowvelocity scanningelectrons will neutralize the charges on mosaic elements i3 until thepositive charges on these elements are neutralized to charge of themosaic elements exceed about 5 volts, whereby the imvelocity of thescanning electrons proportions l5 remains below this value, secondaryemission is practically avoided. The storage eflect, however, is nearlycompletely each other. Therefore, the eiiiciency is only slightly belowbecause complete neutralization of the charges is not advisable, itbeing preferable to discharge only to a residual charge of, forinstance, 10%. Further discharge would not considerably amplify thepicture signal. It would, however, increase the noise level of the tubebecause the main portion of the photocurrent produced at portions l5would then be collected.

It is preferable to avoid too high a photosensitivity on the side ofmesh the insulating layer because the noise level of current is thenreduced. For this purpose a mechanical mask can be used duringevaporation of the silver, or during oxidation, respectively, which canbe removed later on.

It may be diflicult to sensitize the portions I5 uniformly over theentire mosaic area. In order to cause uniform photoemission by thescanning light spot, network H can be made of a material which does notoxidize very readily (nickel, molybdenum, platinum). The work functionof the surface will then be considerably higher than that of a silveroxide-caesium layer. Light of a short wave length, particularly ultraviolet light, is then used for generation of scanning electrons, Theluminescent substance is then so chosen that it has its maximumefficiency, or at least emits suflicient light, in the spectral rangefollowing the shortest wave length of the light used in the opticalrange.

instance M mm., in order to avoid light dispersion. If a mosaic plate ofan area of about 9x11 cm. is used, elements l3 must have a size of about0.2x0.2 mm. for 441 lines. The meshsize of network 12 is determinedthereby.

Instead of scanning fluorescent screen ll with a cathode ray beam, afluorescent screen 01 a separate cathode ray tube can also be opticallyreproduced in the plane of network l2. A pattern of uniform brilliancyis recorded on the screen of this tube in the known manner so thatphotoelectrons are emitted from portions l5 point by point. Fluorescentscreen I can then be omitted.

We claim:

1. A television signal generating device comprising a plurality ofphotoelectric sources, means for developing a spot of light ofsubstantially constant brightness, means for efiecting movements of saidspot across said source in accordance with a scanning pattern to causesaid sources to emit electrons in successive order, photoelectric mosaicmeans adapted to have an optical image focused thereon for collectingportions of the electrons emitted from each of said sources inaccordance with variations in the illumination over said image, andmeans for collecting the remainder of said emitted electrons to producea signal representing said image.

2; A television signal generating device comprising a plurality ofphotoelectric sources, means for developing a spot of light ofsubprising a stantially constant brightness, means for scanning saidsources by said light spot in accordance with a scanning pattern tocause said sources to emit electrons in successive order, photoelectricmosaic means adapted to have an optical image focused thereon forcollecting portions of the electrons emitted from each of said sourcesin accordance with variations in the illumination over said image, andmeans for collecting the remainder of said emitted electrons to producea signal representing said image.

3. A television signal generating device comtranslucent insulatingmember, a coating of fluorescent material on one side of said member,means facing said one' side for generating an electron beam, means fordeflecting said beam across said fluorescent coating in accordance witha scanning pattern, an apertured metallic screen having photosensitiveareas adjacent the other side .of said member, photoelectric mosaicelements disposed on said other side of said member within the aperturesof said screen and adapted to have an optical image focused thereon froma point facing said other side, and means for collecting electronsemitted from said photosensitive areas and said mosaic elements toproduce a signal representative of said image.

ROLF M6LLER. WERNER HARTMANN.

